1. Field of the Invention
The present invention relates to a sound absorbing structure constituted by a porous member. More particularly to a sound absorbing structure for use in an engine cover for an automobile, a sound insulating cover or the like.
2. Description of the Related Art
In general, a fact has been known that a porous member such as open-cell foam and glass wool has a satisfactory sound absorbing characteristic. Therefore, the porous member has been used to absorb sound, for example, in the engine cover or the bonnet of an automobile in order to eliminate noise propagated from an automobile. However, the thickness of the porous member constitute by the open-cell structure must be enlarged to raise the sound absorption coefficient in the midrange and the bass range. Since the space in the inside portion of the engine cover or in the inside portion of the bonnet is limited, there are many cases that the thick sound absorbing member cannot be disposed. Therefore, there are disadvantages such that the conventional porous member having the open-cell structure or continued voids cannot attain a satisfactory sound absorbing effect.
Moreover, a porous member having a cell structure obtained by mixing the open-cells and closed-cells has been employed. In addition, a porous member constituted by an open-cell urethane foam member with a coating film has been also employed as a sound absorbing member. The porous members having the above-mentioned mixed-cell structure and the open-cell urethane foam member with a coating film have a sound absorption peak in a relatively low frequency range. However, a satisfactory high peak value cannot be obtained. As the thickness is increased, the sound absorption peak is shifted to the low frequency range. However, since the width of the peak frequency is too narrow, only when a material having a thickness corresponding to a frequency in a vicinity of a specific and single frequency of a sound source is employed, a somewhat satisfactory sound absorbing effect is sometimes obtained.
However, there are may cases that thickness of the porous members cannot be changed because the structure of the position for use, for example, the inside portion of the engine cover or the inside portion of the bonnet, restricts the thickness. In general, noise from the engine room of an automobile has a frequency range having a somewhat large width. The porous member having the mixed-cell structure cannot attain a satisfactory sound absorbing effect because the width of the peak frequency of the sound absorption coefficient is too narrow and the peak frequency depends on the thickness.
In addition to the above-mentioned porous members, a porous member having a cell structure constituted by only closed-cells has been also used. For example, sponge and expanded rubber are used as this kind of porous member. However, the sound absorption coefficient is too low in the overall frequency range. Therefore, substantially sound absorbing effect cannot be obtained.
Also a perforated plate is employed which is a resonant-type sound absorbing structure incorporating an air layer disposed at the rear of a hard board having through holes. Although a usual perforated plate has a somewhat satisfactory sound absorbing characteristic in a single frequency range, the overall sound absorbing characteristic is unsatisfactory. It is known that a urethane open-cell foam or glass wool provided for the rear air layer of the perforated plate enables the sound absorbing characteristic to be improved. The sound absorbing characteristic, however, is unsatisfactory.
For example, a sound absorbing structure has been disclosed in JP-A-9-13943 which has a structure that a sound absorbing base and a perforated skin member are combined with each other. In JP-A-56-157347, a sound absorbing structure has been disclosed which has a structure that a porous member and a perforated film are combined with each other. In JE-A-56-157346, a sound absorbing structure has been disclosed which has a structure that a porous member and a soft resin sheet provided with air chambers are combined with each other. The foregoing sound absorbing structures has a satisfactory sound absorbing effect in only a specific frequency range. In only a case where a frequency range of noise which is actually made and a frequency range in which the sound absorbing effect is obtained cannot coincide wit h each other, noise can be reduced. The frequency range in which the sound absorbing effect can be obtained cannot arbitrarily be controlled. Therefore, there arises a problem in that noise reduction cannot usually be achieved. To enhance the sound absorbing effect of the foregoing sound absorbing structure, the thickness of the sound absorbing structure must be enlarged. However, if the sound absorbing structure having a large thickness cannot be provided owing to a space limit, noise cannot be reduced.
Hereinafter, the above-mentioned porous members including open-cells and/or closed-cell are generally called also as xe2x80x9cfoam members.xe2x80x9d
In view of the foregoing, an object of the present invention is to provide a sound absorbing-structure and a sound insulation cover having a satisfactory sound absorbing characteristic over a wide frequency range and improving the sound absorbing characteristic in a required frequency range to meet a purpose or the like and the total sound absorption coefficient in a wide frequency range. In addition, it is also an object of the present invention to provide an engine cover incorporating the sound absorbing structure.
The inventors of the present invention have energetically performed studies. As a result, it has been found that a special process to which a foam member having a specific cell structure enables a satisfactory sound absorbing effect to be obtained in a wide frequency range. Moreover, the sound absorbing characteristic in a required frequency range can easily be improved. Moreover, it has been found that an equivalent sound absorbing characteristic can be attained even if the thickness is half or smaller as compared with the sound absorbing material constituted by the conventional foam member. Thus, the present invention is established on the basis of the foregoing facts.
That is, according to the present invention, there is provided a sound absorbing structure comprising a foam member having a cell structure formed by mixing open-cells and closed-cells, wherein a plurality of through holes, which penetrate through the foam member, and/or non-through holes, which do not penetrate through the foam member, with a specific diameters, shapes and depth are formed in the foam member and an engine cover for an automobile incorporating the sound absorbing structure.
Moreover, a structure has been found in which foam members each having a specific cell structure are stacked in a specific state. This structure enables a satisfactory sound absorbing effect to be obtained over a wide frequency range. Moreover, the sound absorbing characteristic in a required frequency range can be easily improved. As compared with a sound absorbing material constituted by a conventional foam member, an equivalent sound absorbing characteristic can be obtained if the thickness is half or smaller. Thus, the present invention has been established on the basis of this facts.
That is, according to the present invention, there is provided a sound absorbing structure comprising two or more stacked layers of foam members each having a mixed-cell structure of open-cells and closed-cells and an engine cover for an automobile incorporating the sound absorbing structure
Furthermore, a special hole formation in a foam member having a specific cell structure enables a satisfactory sound absorbing effect to be realized in a wide frequency range. Moreover, the sound absorbing characteristic can be easily improved in a frequency range having a certain width including a required frequency. As an alternative to this, the sound absorption coefficient in a wide frequency range can totally be improved so that arbitrary control of the sound absorbing characteristic is permitted. Another fact has been found that a sound absorbing characteristic equivalent to the conventional sound absorbing member comprising the foam member can be realized if the thickness is half or smaller. Thus, the present invention is established on the basis of the foregoing facts.
That is, according to the present invention, there is provided a sound absorbing structure comprising a foam member having a cell structure formed by mixing open-cells and closed-cells with one another, wherein the area of opening of each diameter-varied hole formed in the surface opposite to a sound source is largest and the area of opening of each diameter-varied hole is reduced in a direction of the thickness of the foam member and an engine cover for an automobile incorporating the sound absorbing structure.
Moreover, the following fact was found: a structure comprising a coating film formed on at least a surface of a porous member including continued voids which is opposite to a sound source; and through holes penetrating the coating film and the porous member so that the areas of openings of the through holes are adjusted. Thus, the sound absorbing characteristic in a required frequency range can easily be improved. Moreover, the sound absorbing characteristic can easily be controlled. When the sound absorbing structures are laminated, a satisfactory sound absorbing characteristic can be realized in a wide frequency range. As compared with a conventional sound absorbing member comprising a foam member or a molded fibrous material, a sound absorbing characteristic as well as or better than that of the conventional sound absorbing structure can be realized even if thickness of the foam member is not more than half of the conventional one. When the foregoing sound absorbing structure is mounted on a cover, the sound insulation characteristic of the sound insulating cover can be improved. The present invention is established on the basis of the foregoing fact.
That is, according to the present invention, there is provided a sound absorbing structure comprising: a coating film formed on at least a surface of a porous member including continued voids which is opposite to a sound source; and through holes penetrating the coating film and the porous member and a sound insulating cover comprising the sound absorbing structure.